Plate columns weeping

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

VAPOR PRESSURE DROP. The flow of vapor through the holes and the liquid on the plate requires a difference in pressure. The pressure drop across a single plate is usually 50 to 70 mm HjO, and the pressure drop over a 40-plate column is then about 2 to 3 m H2O. The pressure required is automatically developed by the reboiler, which generates vapor at a pressure sufficient to overcome the pressure drop in the column and condenser. The overall pressure drop is calculated to determine the pressure and temperature in the reboiler, and the pressure drop per plate must be checked to make sure the plate will operate properly, without weeping or flooding. 

Technological details and operating characteristics of various types of plate columns are extensively discussed in textbooks on mass transfer (e.g., see Treybal [44]). The results of extensive research on bubble tray design is reported in the American Institute Chemical Engineers Bubble Tray Manual [45]. Flooding and weeping limits and further aspects of sizing are mentioned in a review by Zenz [5]. 

The most important requirement for obtaining satisfactory efficiencies is that the plates operate properly. Adequate and intimate contact between vapor and liquid is essential. Any misoperation of the column, such as excessive foaming or entrainment, poor vapor distribution, or short-circuiting, weeping, or dumping of liquid, lowers the plate efficiency. 

Perforated plates such as sieve trays used in absorption, distillation or extraction columns. The holes can be covered by caps or valves to avoid weeping in the range of low superficial gas or vapor velocities. The two phases are moving in a crossflow on a tray. 

Liquid-phase back mixing is a serious issue for reactions that have nonzero-order kinetics with respect to the liquid-phase reactant. Sectionalization of bubble column using sieve plates of relatively low free area is an attractive choice in such a case. Although this choice has been mentioned in the literature, its application in solid-catalyzed reactions has not attracted any attention. The sieve plate design must be such that it prevents weeping (Prince 1960). The free area in such sieve plates is... 

Thorat BN, Kulkami AV, Joshi JB (2001) Design of sieve plate spargers for bubble columns role of weeping. Chem. Eng. TechnoL, 24 841-828. 

General Consideration. The general arrangement of the plate is shown in Fig 16-13. Weep holes have been added to allow the column to drain when shut down. Three %-in. weep holes were placed in the plate before the exit weir. These should allow the column to drain in about 5 hr. At rated load, the weep holes handle about 2.5 per cent of the total liquid, but even this amount does not short-circuit because it flows down on the proper side. These weep holes can be placed in the inlet and outlet weirs instead of the plate. 

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